In many instances the characteristics of a particular carbonaceous material can be altered by the removal of certain compounds from the material. Examples of some compounds which it may be desirable to remove would include: phospholipids, fats, fatty acids, alcohols, waxes, gums, stearols, oil soluble proteins, flavonol, mineral oils, essential oils, and PCB's.
More particularly, oils derived from plant materials, such as oil-seeds, cereal brans, fruits, beans, and nuts, are the source of raw material for many important commercial products. For example, such oils from such plant materials are extensively used in cooking, low fat and fat free cooked food, in cosmetics, pharmaceuticals as carriers for insecticides and fungicides, in lubricants, and in myriad other useful products. Consequently, much work has been done over the years in developing improved processes for extracting oil from such materials.
One of the most widely used processes for removing oil from oil-bearing materials is solvent extraction. In solvent extraction, the oil-bearing material is treated with a suitable solvent, usually the lower carbon alkanes such as hexane, at elevated temperatures and low pressures, to extract the oil from the oil-bearing material. The resulting solvent/oil mixture is then fractionated to separate the valuable oil from the solvent, which is recycled. Most solvent extraction processes in commercial use today employ hexane as the solvent. While hexane extraction is the most widely used today, there are also teachings in the art in which normally gaseous solvents are used at both supercritical and subcritical conditions.
One such teaching is found in U.S. Pat. No. 1,802,533 to Reid, wherein a normally gaseous solvent, preferably butane or isobutane, is liquefied by decreasing the temperature and/or increasing the pressure, then passing the solvent through a bed of the oil-bearing material in an extraction vessel. The solvent and extracted oil are then passed to a still where the solvent is separated from the oil. The extracted material must then be placed in another still where it is heated to remove solvent which remained entrained in the extracted material. There is no suggestion of obtaining a substantially solvent-free, dry, extracted material without an additional treatment step after extraction.
Another extraction process is taught in U.S. Pat. No. 2,548,434 to Leaders wherein an oil-bearing material is introduced into the top of an extraction tower and passed counter-current to a liquefied normally gaseous solvent, such as propane, which is introduced at the bottom of the extraction tower. The tower is operated near critical conditions so that the solvent selectively rejects undesired color bodies, phosphatide, gums, etc. The resulting solvent/oil mixture can then be flashed to separate the solvent from the oil. In another embodiment, the solvent/oil mixture is first subjected to a liquid/liquid separation resulting in one fraction containing solvent and a less saturated fatty material, and another fraction containing solvent and a more saturated fatty material. The solvent is then flashed from both fractions. The extracted material remaining in the tower is drawn off and subjected to a vacuum flashing operation to remove entrained solvent.
U.S. Pat. No. 4,331,695 to Zosel teaches a process for extracting fats and oils from oil-bearing animal and vegetable materials. The material is contacted with a solvent, such as propane, in the liquid phase and at a temperature below the critical temperature of the solvent to extract fat or oil from the material. The resulting solvent/oil mixture is treated to precipitate the extracted fat or oil from the solvent by heating the solvent to above the critical temperature of the solvent without taking up heat of vaporization. The extracted residue (shreds) is then treated to remove any entrained solvent, either by blowing it directly with steam, or by indirect heating followed by direct steaming.
In U.S. Pat. No. 5,041,245 to Benado a continuous solvent extraction method utilizing propane is disclosed to remove oils from vegetable matter, particularly rice bran. According to this method a sufficient amount of liquid sealing medium is first injected into the vegetable matter in a feeding zone to form a dough-like plastic mass which is compacted and transported by a conveyor assembly to an extraction zone to form a bed. Propane is then introduced into the bed of the extraction zone being operated at 102.degree.-122.degree. F. and 125-250 psi to react with the bed material. The miscella of extracted oil and solvent resulting from this from the reaction of propane and bed material is then separated from the remaining solid residue of the bed material. The propane is then separated from the extracted oil by evaporation or volatization methods. The preferred separation method is to first subject the miscella to near its critical pressure (600 psi for propane/rice bran oil mixture) and significantly elevated temperatures (190.degree.-200.degree. F. for propane/rice bran oil mixture) which can also be near critical. This yields a high solvent light phase (98% solvent, 2% bran oil) and an oil-enriched heavy phase (60% solvent, 40% bran oil). The oil enriched heavy phase under reduced pressure is then delivered to a heater-evaporator and further treated to form a more oil-enriched heavy phase (10% solvent, 90% bran oil). This phase is then de-pressurized to about one atmosphere, and further treated in a second combined heater-evaporator stage to produce an oil stream having not more than 1-2% propane. Further similar treatment of this oil stream could be accomplished to remove additional propane if desired.
Other references which teach solvent extraction of oil-bearing materials, with normally gaseous solvents, include U.S. Pat. Nos. 2,682,551 to Miller; and 2,560,935 to Dickinson. In each of these processes, the extracted material must be further processed to remove entrained solvent.
While prior art extraction methods, particularly hexane extraction, have met with various degrees of commercial success, there still remains a need in the art for an improved solvent extraction method which is more energy and cost efficient, which can effectively remove the solvent from the extracted compounds to meet government regulations, which is especially suitable for the processing of certain troublesome oil-bearing materials, as well as which allows greater selectivity of the compounds removed from the carbonaceous material and which results in the recovery of de-oiled products having superior nutrient and health characteristics.
In solvent extraction of oil from carbonaceous materials, such as vegetable material, one problem has been fluidization problems in the bed formed by the material in the extraction vessel. This has lead to the need to pre-pelletize or compact the material before placing the material in the extraction vessel to increase the material bed permeability and allow the solvent to penetrate and flow through all portions of the material bed. This problem is particularly acute in those situations where a significant amount of the vegetable material are of small particle size, e.g., 100 to 400 mesh.
When the carbonaceous material contains significant amounts of oil, current solvent extraction methods have been inefficient for removing most or all of the oil. Examples of such material would include jojoba, cocoa, rape seed, and canola which are 30%-60% by weight oil. In these instances it has been necessary to first press the material to remove a majority of the oil before using solvent extraction methods to remove the remaining amounts of oil. Alternatively, the material could be first mechanically ground or pulverized to render the oil more accessible to reaction with the solvent. This latter method is difficult if the material has a high oil content.
In many of the instances where the material must first be pressed it is necessary to subject the material to high temperatures (200.degree.-360.degree. F.) to effectively remove the oil. In food material such high temperatures can result in deleterious effects to the desirable characteristics of the material, such as protein denaturing, vitamin destruction, and creation of carbonic acids which effect the aromatic odor of food material such as spices and herbs.
One particularly troublesome material is rice bran, one of the most plentiful and nutritious food sources known to man, but which is greatly under utilized. This is primarily because immediately following the milling step, a lipolytic enzyme in the bran is activated which catalyzes the hydrolysis of the glyceryl esters of the free fatty acids (FFA) present in the lipids. This is measured by FFA increase, which is rapid at typical atmospheric storage conditions. This starts fatty acid formation and bran rancidity in a matter of minutes after milling, and eventually renders it inedible to humans after several days of storage. Consequently, rice bran, as a source of oil and food, is under utilized, particularly in less developed countries. While the food industry struggles to find ways to obtain a rice bran, and rice bran oil, free of these undesirable characteristics, more and more beneficial uses and nutritive values are being discovered for these products. For example, it has recently been reported that rice bran fiber is effective for lowering cholesterol in humans. As a result, a tremendous demand has been created for a process which can stabilize the rice bran after milling, or a process which will allow for the extraction of oil while at the same time stabilizing the oil and bran against further fatty acid formation.
Other problems are encountered with different food products. For example, in eggs it is desirable to remove the cholesterol from the yoke, yet have the eggs retain their natural proteins not denatured, texture, and taste when cooked. This has not been possible with the present known methods of solvent extraction.
As another example, in many commercially available seasonings and food coating products one problem has been the inability to remove certain fats while retaining the flavoring of the products.
Still another problem has been to create seasoned or unseasoned food coatings that have dielectric characteristics which increase the ability of the coating to adhere to the food product during handling and cooking. A further problem with food coatings occurs when the food product is mircowaved. The moisture in the food product permeates the coating during the cooking process resulting in a soggy, unappetizing-looking crust.
Still other problems occur when trying to remove oils and fats from fried products such as potato chips and french fries. Current methods result in undesirable flavor or texture changes because of the inability of these methods to selectively remove only the undesired compounds.
The treatment of animal products by present solvent extraction processes to remove fats and cholesterol have not been commercially successful because of the dilatory effect on the taste, color or texture characteristics of the cooked animal products.